Field of the Invention
This invention relates to latch needles (hereinafter referred to as being of the kind specified) comprising an elongate shank portion, a hook portion at one end of the shank portion, a latch element pivotally mounted on the shank portion for movement between closed and open positions with respect to the mouth of the hook portion, and a butt portion spaced along the shank portion from the hook portion.
The shank portion and the butt portion of a latch needle of the kind spcified is normally made of strip metal in the form of flattened metal wire and the side faces of the shank portion have depths which are greater than the thickness of the shank portion. In the following description, for convenience, it will be assumed that the side faces of the needle lie in vertical planes and that the edge faces of the needle are approximately horizontal, the hook portion is at the forward end and the butt portion is at or adjacent to the rearward end of the shank portion.
One of the principal applications of needles of the kind specified is in knitting machines. In such an application the needles may be mounted in grooves or, as they are termed, tricks, which extend longitudinally in the outer surface of a mounting cylinder for the needles, and the latter are reciprocated longitudinally in the tricks by cooperation between the needle butts and a cam.
For high productivity it is a requirement that the knitting machine shall be operated at the highest speed consistent with satisfactory knitting operation and service life of the component parts of the machine, but one of the limitations in this respect is failure of the needles which often takes place by fracture of the hook portion with or without part of the shank portion attached from the shank portion or the remainder of the shank portion.
It is known that this particular form of failure increases with increasing working speed of the knitting machine. Many studies have been made and experiments conducted in an endeavour to analyse or ascertain the cause or causes of this mode of failure with a view to developing solutions without any large measure of success having been achieved.
The present invention is based upon recognition that the principal cause of needle failure in the mode referred to is excitation of shock waves originating at the butt of the needle as a result of impact between the butt and the operating cam, and the reflection of such waves from the end of the needle at which the hook portion is situated. When a shock wave is reflected as aforesaid it travels reversely along the shank portion towards the butt and at some position along the needle spaced from the end of the hook portion, the needle is subjected simultaneously to shock stress arising from the leading end of the reversely travelling shock wave and the trailing end of the forwardly travelling shock wave, the stresses respectively being of opposite kind, e.g. compressive and tensile, so that they are additive.